1. Field of the Invention
The present invention relates to tire sensors for monitoring vehicle tire performance including tire traction, and more particularly, to a three-axis sensor assembly including a plurality of individual sensor elements disposed in a pyramidal arrangement for use in a vehicle tire.
2. Description of the Related Art
Advances in computer processing power have enabled many improvements to automotive technology in the form of automatic control of vehicle parameters. Engine functions have largely come under computer control with the aid of sensors incorporated into the engine. Advanced and inexpensive sensors and electronics have also enabled traction control and anti-lock braking.
One known system utilizes data pertaining to a tire contact patch in conjunction with driver input and vehicle motion sensors to control the movement of a motor vehicle. The system requires input from a sensor means in the vehicle tire which provides information about changes in the tire footprint. Typically, toroidal bands of either piezoresistive or piezoelectric material incorporated in the rubber of the tire as a transducer of strains present in the tire carcass are void. Notably, the measurement obtained by this device is not localized to a signal tread block, and as a result, suffers from undesirable effects due to centrifugal force, road surface irregularities, and pressure changes.
In another device, reed sensors incorporating strain gauges are employed, each sensor measuring forces directed in a single axis. In this arrangement, three separate devices, disposed at three separate locations, are required to obtain three axes of traction data. A significant problem associated with such a device is that each individual tread block will experience forces from the three axes concurrently. Typically, each tread block acts independently in a stick-slip fashion. As a result, measuring X axis data from one tread block, Y axis data from an adjacent tread block and Z axis data from yet another location, will yield three axes of data that is of little use.
Yet another system includes using a magnetic sensor for determining tire-road adhesion. This device determines the strain within the tire rubber indirectly by measuring a displacement of the sensor from a reference point such as a magnetized region of the steel belt. A more direct measurement of strain would give a better determination of tire-road adhesion than a displacement measurement.
Such a device also has the disadvantage of being difficult to manufacture. It requires that the tire be x-rayed after it is manufactured so as to locate the magnetic sensor. Then the steel belts must be subsequently magnetized in the proximity of the embedded sensor.
Conventional strain gauges or strain sensors are typically used for measuring the expansion and/or contraction of an object under stress. Strain sensors may comprise a resistive transducer, the electric output of which is proportional to the amount it is deformed under strain. In one type of resistive strain gauge illustrated in FIG. 1, the gauge 1 is made of a metal foil or metal wire 2 that is mounted on a substrate 3, wherein the wire changes resistance with expansion or contraction in a particular direction. FIG. 1 illustrates movement of the gauge, which is indicative of movement of the object being monitored, with the arrow xe2x80x9cxxe2x80x9d indicating movement in the xe2x80x9cxxe2x80x9d direction. Such a sensor requires either a DC or an AC excitation voltage to generate a strain signal. In addition, it is preferably connected in a differential arrangement such as in a Wheatstone bridge circuit to determine the amount of strain. Other types of strain sensors include parallel plate capacitors, piezoresistive silicon strain gauges, piezoelectric devices such as lead zirconium titanate (PZT), capacitors formed of inter-digitated fingers simulating adjacent parallel-plate capacitors, conductive elastomer resistive strain gauges, etc.
Each of these strain sensors is adapted to measure strain forces exerted on an object in a particular direction. However, multiple axis strain detection is of particular concern for the present invention in determining shear and compressive strain in an elastomeric tire. Monitoring the forces exerted on the tread rubber of a tire in multiple axes can provide an indication as to the performance of the tire (e.g., traction), as well as provide information valuable, for example, in controlling different components of a vehicle. According to one type of tire monitoring sensor, the deflection of tire tread is measured as it passes through a contact patch, the contact patch being defined by that portion of the tire in contact with the road at any particular time. The sensor in this device is a piezoelectric polymer, a photo restrictive fiber optic, a variable plate capacitor, or a variable inductor, each of which is capable of measuring the length of the contact patch during tire operation. In addition, the sensor is connected to a transponder device for communicating single-axis strain data for analysis. Most notably, the data obtained by such a sensor does not provide any useful traction information because it is only capable of measuring the length of the contact patch. As a result, variables which affect the coefficient of friction, such as road condition, are ignored. Overall, this sensor is unable to provide sufficient data for determining tri-axial strain forces of interest.
In general, traction control and anti-lock braking systems fall short of optimal computer control of vehicle handling. What known systems lack in this area is an effective real-time, in-situ measurement of the traction at each individual vehicle tire.
As a result, the field of traction sensing was in need of a sensor assembly that measures strain in three dimensions at a particular point or region so as to monitor tire traction more effectively. Moreover, such a device should be self-contained and be capable of being embedded in an object to be monitored, such as an elastomeric material (e.g., the rubber of a tire), during manufacture of the object without compromising the integrity of its performance.
The present invention is directed to a sensor assembly capable of measuring X-Y shear strain and compressive Z strain while embedded in the tread rubber of a vehicle tire. The preferred embodiment is adapted to being embedded during the manufacture of the tire, and is capable of measuring strain in three axes independent of the particular location within the tire tread. The sensor assembly can be a self-contained device, and as such is well-suited to making three axis measurements at a particular point or region of the tire being monitored. Moreover, the assembly is particularly adapted to generate data to determine tire traction between the footprint of the vehicle tire and the corresponding road surface, whereby one or more of the assemblies can be connected to a transponder to communicate the data to, for example, the vehicle on which the tire is mounted.
According to one aspect of the preferred embodiment, a tire sensor assembly is embedded in an elastomeric tire at a particular radial depth inwardly from a contact patch of the tire. The sensor assembly includes a flexible generally pyramid-shaped body and a pair of first strain sensors disposed on first opposed faces of the pyramid-shaped body, the first strain sensors detecting a force in a first direction. In addition, the assembly includes a pair of second strain sensors disposed on second opposed faces of the pyramid-shaped body, the second strain sensors detecting a force in a second direction. Moreover, each face of the first and second opposed faces is non-planar.
In another aspect of this embodiment, the first strain sensors of the sensor assembly generate corresponding output signals in response to the force in the first direction, the force in the first direction being generally equal to the difference between the output signals of the first strain sensors. Also, the second strain sensors generate corresponding output signals in response to the force in the second direction, wherein the force in the second direction is generally equal to the difference between the output signals of the second strain sensors.
In a further aspect of this embodiment, the first and second opposed faces of the pyramid-shaped body are curved and generally symmetrical about an axis extending longitudinally through the apex of the body so as to allow adjustment of the sensitivity of the sensor assembly generally independent of the radial depth.
In yet another aspect of this embodiment, the sensor assembly is more sensitive to a tensile strain and less sensitive to a shear strain, and the first and second opposed faces are concave.
Another aspect of the preferred embodiment is a method of embedding a sensor in a tire, which includes providing a three-axis sensor assembly having two pairs of strain gauges. A first pair of strain gauges is disposed on first opposed faces of a generally pyramid-shaped body, and a second pair of strain gauges is disposed on second opposed faces of said generally pyramid-shaped body. The method further includes the step of shaping the first and second opposed faces of the pyramid-shaped body so that each face of the first and second opposed faces is non-planar.
In yet another aspect of the preferred embodiment, the first and second opposed faces of the pyramid-shaped body are curved, thus causing the desired sensitivity to be greater in response to a tensile strain and less in response to a shear strain. In addition, the shaping step includes shaping the first and second opposed faces of the pyramid-shaped body so that the faces are concave.
Another aspect of the preferred embodiment is that the desired sensitivity is characterized by greater sensitivity to a shear strain and less sensitivity to a tensile strain. In this aspect, the shaping step includes shaping the first and second opposed faces of the pyramid-shaped body so that the faces are convex.
These and other objects, features, and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.